Continuously variable transmission (CVT) has long been recognized as a likely answer to more efficient automatic transmissions but CVT has been relatively slow in gaining widespread commercial use. In Business Week, Feb. 6, 1984 at pages 74-75 there is discussed the basic approach to a currently popular form of CVT. As described therein CVT works by varying the working diameters of two pulleys, each of which is characterized by a V-shaped groove in which sits the pulley belt. To shift into a lower gear, the engine pulley spreads apart dropping the belt lower into its V-shaped groove and at the same time, the driven shaft pulley closes, forcing the belt to ride higher in its V-groove, thereby effectively decreasing the ratio of the diameters of the engine pulley and the drive shaft pulley.
A critical element of such a system is the belt used. Typically there is used a flexible steel belt with hundreds of steel plates strung together like the links of a watch band. In such a system, the drive shaft pulley pushes the driven shaft pulley into rotation by means of the segments of the belt which are pinched between the sidewalls of the respective groove.
An oil film is included between the walls of the groove formed by the pulley walls and the tips or edges of the steel belt to facilitate displacement of the belt when a speed change is made. Unfortunately, this oil film requires a very high contact force to enable the system to transfer torque between the pulleys and the drive belt. It is of course important that adequate clamping be maintained between the belt and the pulleys if either the pulley is to drive the belt or be driven by the belt without slipping. In particular, it is advantageous that the clamping force be proportional to the torque being transmitted. Moreover, it is important that these high clamping forces be little affected by shock or other load changes. Prior art CVTs have been deficient in these respects.